The present invention relates to an image forming apparatus which forms an electrostatic latent image onto a latent image carrier by using writing electrodes of a writing device, thereby forming the image.
In a conventional image forming apparatus such as an electrostatic copier and a printer, the surface of a photoreceptor (photosensitive member) is uniformly charged by a charging device and the charged surface is then exposed to light from an exposure device such as laser beam or LED light, whereby a latent image is written on the surface of the photoreceptor. Then, the latent image on the surface of the photoreceptor is developed by a developing device to form a developing powder image on the surface of the photoreceptor. The developing powder image is transferred to a receiving medium such as a paper, thereby forming the image.
In such conventional image forming apparatus, the exposure device as a writing device for electrostatic latent image comprises a laser beam generating device or a LED light generating device. Therefore, the entire image forming apparatus should be large and complex.
Therefore, an image forming apparatus has been proposed in Japanese Patent Publication No. S63-45104 (hereinafter, ""104B publication) which employs electrodes, as a writing device for forming an electrostatic latent image, to write an electrostatic latent image on a surface of a latent image carrier without using laser beams and LED lights.
The image forming apparatus disclosed in the ""104B publication is provided with a multistylus having a large number of needle electrodes. The needle electrodes are just arranged in contact with an inorganic glass layer on the surface of the latent image carrier. In accordance with an input signal for image information, voltages are selectively applied to corresponding ones of the needle electrodes of the multistylus, whereby the electrostatic latent image can be formed on the latent image carrier. Since the image forming apparatus according to the ""104B publication does not use an exposure device conventionally used as a writing device, the invention of this publication can provide an image forming apparatus which is relatively small in size and relatively simple in structure.
In addition, an image forming apparatus has been proposed in Japanese Unexamined Patent Publication No. H06-166206 (hereinafter, ""206A publication), comprising ion control electrodes which are disposed on a front end portion of an insulating substrate and are arranged in non-contact with a latent image carrier, wherein the ion control electrodes control ions produced by a corona discharger so as to write an electrostatic latent image on the latent image carrier. Since the image forming apparatus according to the ""206A publication also does not use an exposure device as a writing device, the invention of this publication can provide an image forming apparatus which is relatively small in size and relatively simple in structure.
However, in the image forming apparatus according to the ""104B publication, the large number of needle electrodes of the multistylus are just arranged in contact with the inorganic glass layer on the surface of the latent image carrier. It is difficult to keep the stable contact between the needle electrodes and the inorganic glass layer on the surface of the latent image carrier. Accordingly, it is difficult to stably charge the surface of the latent image carrier. This means that it is hard to obtain a high quality image.
Moreover, it is unavoidable to employ an inorganic glass layer on the surface of the latent image carrier for protecting the surface of the latent image carrier from damage due to contacts of a large number of the needle electrodes. This makes the structure of the latent image carrier more complex. In addition, since the inorganic glass layer has quite well physical adsorbed water characteristic, moisture is easily adsorbed by the surface of the inorganic glass layer. Due to the moisture, the electrical conductivity of the glass surface is increased so that electrostatic charge on the latent image carrier should leak. Therefore, the image forming apparatus should be provided with a means for drying the surface of the latent image carrier with adsorbed moisture in order to prevent the apparatus from being affected by absorbed water. This not only makes the apparatus larger but also increases the number of parts, leading to problems of making the structure further complex and increasing the cost.
Since the large number of needle electrodes discharge, the apparatus has another problem that there is a high possibility of generation of ozone (O3). The presence of ozone may not only produce rusts on parts in the apparatus but also melt resin parts because ozone reacts with NOx to generate nitrous acid (HNO3). Again ozone may give an offensive smell. Therefore, the image forming apparatus should be provided with an ventilation system including a duct and an ozone filter which sufficiently exhausts ozone from the inside of the apparatus. This also not only makes the apparatus larger but also increases the number of parts, leading to problems of making the structure further complex and increasing the cost.
On the other hand, in the image forming apparatus according to the ""206A publication, ions produced by the corona discharger are controlled by the ion control electrodes. This means that the apparatus is structured not to directly apply electric charge to the latent image carrier. The invention of the ""206A publication has problems of not only making the image forming apparatus larger and but also making the structure complex. Since the charge is conducted by ions, it is difficult to stably write a latent image on the latent image carrier.
Further, since the generation of ions essentially generates ozone, there are problems similar to those described with regard to the image forming apparatus according to ""104B publication.
It is an object of the present invention to provide an image forming apparatus capable of more stably writing an electrostatic latent image and yet achieving reduction in size and reduction in the number of parts thereof so as to have more simple and low-price structure.
It is another object of the present invention to provide an image forming apparatus capable of further preventing generation of ozone.
In order to achieve these objects, the present invention provides an image forming apparatus which comprises at least: a latent image carrier on which an electrostatic latent image is formed, a writing device for writing said electrostatic latent image on said latent image carrier, and a developing device for developing said electrostatic latent image on said latent image carrier, wherein said electrostatic latent image, written on said latent image carrier by said writing device, is developed by said developing device, thereby forming an image, and is characterized in that said writing device has writing electrodes which are in plane contact with a charged layer of said latent image carrier to write said electrostatic latent image.
The present invention is characterized in that said writing electrodes are supported by a substrate having elasticity and are kept in contact with said latent image carrier by a small pressing force due to the elasticity of said substrate.
Further, the present invention is characterized in that said writing electrodes write said electrostatic latent image on said latent image carrier by applying charge to said latent image carrier.
Furthermore, the present invention is characterized in that said writing electrodes write said electrostatic latent image on said latent image carrier by removing charge from said latent image carrier.
The present invention is still characterized in that said writing electrodes are controlled to be connected to either high voltage or low voltage by switching operation in accordance with the image to be formed, thereby writing said electrostatic latent image on said latent image carrier.
The present invention is still further characterized in that the resistance value of said each writing electrode is set at a value smaller than the resistance value of said charged layer.
Further, the present invention is characterized in that the resistance value of said each writing electrode is set at 108 xcexa9 cm or less.
Furthermore, the present invention is characterized in that the resistance value of said each writing electrode is set at 106 xcexa9 cm or more.
Moreover, the present invention is characterized in that the resistance value of said charged layer is set at 109 xcexa9 cm or less.
The present invention is still characterized in that said each writing electrode is provided with a resistive layer at its conductive portion so that said writing electrode is formed in a multi-layered structure, wherein said resistive layer of said writing electrode is in contact with said latent image carrier.
The present invention is still further characterized in that said writing device and said developing device are provided for every color of black, yellow, magenta, and cyan, whereby a multicolored developing powder image is formed by superposing respective color developing powder images which are formed by said writing devices for the respective colors and said developing devices for the respective colors.
Moreover, the present invention is characterized in that said latent image carrier, said writing device, and said developing device are provided for every color of black, yellow, magenta, and cyan, whereby four image forming units for the respective colors are provided, said image forming units being arranged in tandem.
In addition, the present invention is characterized by further comprising an intermediate transferring device, to which respective color developing powder images are temporally transferred by said image forming units for the respective colors.
In the image forming apparatus of the present invention, charge-transfer between the writing electrodes and the latent image carrier which are in contact with each other are dominant because the writing electrodes are in plane contact with the latent image carrier, thereby stably and reliably conducting the charge-transfer between the writing electrodes and the latent image carrier. This dominant charge-transfer enables easy writing of an electrostatic latent image on the latent image carrier.
In the application or removal of charge via charge-transfer, since charge is directly transferred between the writing electrodes and the latent image carrier which are in contact with each other, the surface potential of the latent image carrier becomes substantially equal to the voltage to be impressed to the writing electrodes. Therefore, the voltage to be impressed to the writing electrodes is allowed to be set at a relatively low voltage.
The writing electrodes are supported by the substrate having elasticity and are kept in contact with the latent image carrier with a small pressing force by weak elastic restoring force of the substrate, thereby stabilizing the positions of the writing electrodes relative to the latent image carrier and thus stably and reliably conducting charge-transfer between the writing electrodes and the latent image carrier. Therefore, application or removal of charge relative to the latent image carrier by the writing electrodes can be further stably conducted with high precision, thereby achieving stable writing of an electrostatic latent image and thus reliably obtaining a high quality image with high precision.
Since the writing electrodes can be kept in contact with the latent image carrier with a small pressing force as mentioned above, there is little or no gap (space) between the writing electrodes and the latent image carrier. Because of the little or no gap, air undesirably ionized is practically non-existent, thereby further reducing the generation of ozone and enabling the formation of an electrostatic latent image with low potential. Since the writing electrodes are kept in contact with the latent image carrier by a small pressing force, the latent image carrier can be prevented from being damaged by the writing electrodes, thus improving the durability of the latent image carrier.
Further, since the resistance of the writing electrode is set to be a value equal or less than the resistance of the charged layer of the latent image carrier, the speed of charge response during latent image formation is hard to be affected by resistive components adhering to the surface layer of the writing electrode because the resistance of the charged layer is greater than the resistance of the writing electrode.
By setting the resistance value of the writing electrode at 108 xcexa9 cm or less, a predetermined time constant can be ensured, thus achieving uniform charge. On the other hand, by setting the resistance value of the writing electrode at 106 xcexa9 cm or more, the electrostatic breakdown due to pin holes of the charged layer of the latent image carrier can be prevented. It should be understood that this lower limit of the resistance may be lower if a blocking layer (thin insulating layer) is provided on the latent image carrier.
The setting of the resistance of the charged layer at 109 xcexa9 cm or less can facilitate application of charge which is conducted through the contact charge-transfer between the electrode and the latent image carrier which are in contact with each other.
Furthermore, the writing electrode is provided with the resistant layer on the conductive portion, thereby preventing the broadening of the contact charge-transfer in the lateral direction. This achieves effective contact charge-transfer between the writing electrode and the latent image carrier which are in contact with each other.
Moreover, the writing device employs only the writing electrodes without using a laser beam generating device or a LED light generating device which is large in size as conventionally used. The apparatus size can be reduced and the number of parts can also be reduced, thereby obtaining an image forming apparatus which is simple and low-price.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.